Reducing the number of pins required by an integrated circuit is an important aspect for reducing the size and cost of the integrated circuit. This is also true for monaural audio integrated circuit devices with a serial pulse code modulated (PCM) audio interface. These serial audio interfaces often have multiple channels multiplexed to a serial data stream. In a stereo audio environment, two audio signals (“left” and “right”) are digitized and then time-multiplexed for transmission over the same interface. The audio integrated circuit device has at least one additional pin that is used to select the audio integrated circuit device as either a “left” channel device or a “right” channel device. The number of input/output pins on integrated circuits that have a relatively low number of pins, impacts the size of the integrated circuit and the package into which the integrated circuit is mounted. Having fewer pins can improve the size and cost of the audio integrated circuit.
An example of a serial audio interface is the “Integrated Inter-chip Sound” (I2S) developed in 1986 and revised in 1996 by Philips Semiconductor N.V. (NXP Semiconductor N.V., Eindhoven, the Netherlands) and is well known in the art. The I2S bus is designed for the transfer of only audio data, while the other signals, such as sub-coding and control, are transferred separately. To minimize the number of pins required and to keep wiring simple, a 3-line serial bus is used. Referring to FIG. 1, the I2S bus consists of a serial data SD line that has two time-multiplexed data channels (LEFT CHANNEL and RIGHT CHANNEL), a word select WS line for designating the channel boundaries, and a clock line SCK for synchronizing the transmission and reception of the data. The two time-multiplexed data channels (LEFT CHANNEL and RIGHT CHANNEL) are transmitted sequentially.
Since the transmitter and receiver have the same clock signal SCK for data transmission, the transmitter, as the master, has to generate the bit clock SCK, word-select signal WS and the serial data SD. In complex systems however, there may be several transmitters and receivers, which makes it difficult to define the master. In such systems, there is usually a system master controlling digital audio data-flow between the various ICs. Transmitters then, have to generate the serial data SD under the control of an external clock SCK and an external word-select signal WS. The transmitters then act as a slave.
The serial data is transmitted in two's complement pulse coded modulation (PCM) with the most significant bit (MSB) first. The MSB is transmitted first because the transmitter and receiver may have different word lengths. It isn't necessary for the transmitter to know how many bits the receiver can handle, nor does the receiver need to know how many bits are being transmitted. When the system word length is greater than the transmitter word length, the word is truncated (least significant data bits are set to ‘0’) for data transmission. If the receiver is sent more bits than its word length, the bits after the LSB are ignored. On the other hand, if the receiver is sent fewer bits than its word length, the missing bits are set to zero internally. And so, the MSB has a fixed position, whereas the position of the LSB depends on the word length. The transmitter always sends the MSB of the next word one clock period after the word select signal WS changes. Serial data SD sent by the transmitter may be synchronized with either the trailing (HIGH-to-LOW) or the leading (LOW-to-HIGH) edge of the clock signal SCK. However, the serial data must be latched into the receiver on the leading edge of the serial clock signal.
The word select line WS indicates the channel being transmitted. Where the word select line WS=0, the LEFT CHANNEL is selected and where the word select line=1 the RIGHT CHANNEL is selected. The word select line WS may change either on a trailing or leading edge of the serial block (a string of bits from the most significant bit (MSB) to the least significant bit (LSB), but it does not need to be symmetrical. In the slave, this signal is latched on the leading edge of the clock signal. The word select line WS changes one clock period before the MSB is transmitted. This allows the slave transmitter to derive synchronous timing of the serial data that will be set up for transmission. Furthermore, it enables the receiver to store the previous data packet (n−1, n, n+1) and clear the input for the next word (n−1, n, n+1).
FIGS. 2a-2f are block diagrams of configurations of transmitter devices and receiver devices communicating audio signals on an I2S serial communication interface. In FIG. 2a, a digital signal processor DSP is a master device transmitting the serial bit clock SCK, the word select WS or channel select clock, and the serial data SD to a receiving I2S device acting a slave. In this example, the slave device would be a loudspeaker.
In FIG. 2b, the receiving I2S device is acting as the master device generating and transmitting the serial bit clock SCK and the word select WS. The digital signal processor DSP is the slave device transmitting the serial data SD to the master. In this example, the master device would be the loudspeaker, but would also have the clock generating circuitry for the I2S bus.
In FIG. 2c, the digital signal processor DSP is a master device transmitting the serial bit clock SCK and the word select WS to the I2S device acting a slave. The I2S device transmits the serial data SD to the digital signal processor DSP. In this example, the slave device would be a microphone.
In FIG. 2d, the I2S device is acting as the master device generating and transmitting the serial bit clock SCK, the word select WS, and transmitting the serial data SD to the digital signal processor DSP that is a slave device. In this example, the master device would be the microphone with digitizing circuitry for creating the digitized analog (audio) signal for the serial data bus SD. The I2S device also has the clock generating circuitry for the serial bit clock SCK and the word select clock WS for the I2S bus.
In FIG. 2e, the digital signal processor DSP is the master device transmitting the serial bit clock SCK, the word select WS, and the serial data SDO to a receiving I2S device acting a slave. In this example, the slave device also transmits a serial data SDI to the digital signal processor DSP. In this case, the I2S device may be a combination of a microphone and a loudspeaker in the same unit with the serial data SDI providing the digitized analog signal from the microphone.
In FIG. 2f, the I2S device is the master device transmitting the serial bit clock SCK, the word select WS, and the serial data SDO to the digital signal processor DSP acting a slave. In this example, the digital signal processor DSP also transmits a serial data SDI to the I2S device. In this case, the I2S device may be a combination of a microphone and a loudspeaker in the same unit with the serial data SDI providing the digitized analog signal from the microphone. In FIGS. 2a-2f, the master device is the generator and transmitter of the serial bit clock SCK and the word select WS signals.
A digital audio device, e.g. a loudspeaker or microphone amplifier that is connected using the I2S bus as described above has a limited pin count. It usually contains only the I2S bus and power supply pins. In order to configure the digital audio device, a separate command bus using an interface such as the Inter-Integrated Circuit (I2C) bus maybe added to the audio device. However, this added circuitry requires more pins, consumes more power and occupies more space causing the physical dimensions of the audio device to grow. Interfaces such as the I2C generally require a number of bits that are used as a preamble that is transmitted before an actual audio stream may start. This has a disadvantage in that the digital audio device can only be programmed once in an audio stream transmission. If a change in the settings is required during the audio stream transmission, the audio stream transmission will be forced to stop. This stopping of the audio stream transmission is very audible.